When can we secure a 4-room flat or pay the option fee during SBF, and is it dependent on the queue number?

You can secure a 4-room flat only when you are shortlisted and invited to book a unit. At that point, you must pay the option fee of $2,000. Whether you get a chance to select and pay for a 4-room flat depends on your queue number, as availability is affected if your turn exceeds the number of flats offered.